Thermal Expansion of Pure Nickel and Cobalt. 161 



gills atrophy later. During the process of their doing so, the Lepido- 

 siren passes through a condition in which the stumps persist evidently 

 corresponding to that well known in the young Protopterus, the group 

 of external gills with their common stalk having come by differential 

 development to be situated immediately above the fore limb. After 

 the close of the larval period the Lepidosirens become much darker in 

 colour and more lively in their movements. Young were obtained from 

 the nest up to a length of 60 mm. About this time the cornea begins 

 to assume the white unhealthy appearance that it has in the adult. In 

 the young of this size, small yellow spots appear, and in the young of 

 90 mm. these are conspicuous. Occasional yellow blotches persist in the 

 young Lepiclosiren of eighteen months, but in the adult they disappear. 



The paper concludes with general remarks on the phenomena de- 

 scribed. The segmentation approaches most closely that of Ganoids. 

 The shortening up of the invaginating groove is considered to illus- 

 trate a process which has taken place in phylogeny in the passage from 

 the primitive holoblastic egg to the meroblastic condition. The con- 

 tinuity of the medullary folds behind the anus is adduced, together 

 with the evidence accumulating of the prolongation of the blastopore 

 along the floor of the medullary groove in other forms (Amphibia, 

 Ceratodus, e.g.) as affording potent evidence in favour of the hypothesis 

 which derives the Vertebrata from ancestral forms as primitive as the 

 Ccelenterata, and possessing a nelorigated mouth traversing the neural 

 surface. The occurrence of external gills in the young of three so 

 comparatively primitive groups of . Vertebrata as Crossopterygians, 

 Dipnoans, and Amphibians ; their occurrence on four branchial arches 

 in Lepidosiren, and on at least the hyoicl arch in Crossopterygians, and 

 the occurrence of a probable homologue on the mandibular arch in 

 Urodela, are taken as suggesting that these structures are organs of 

 great antiquity in the Vertebrate stem, and that there was formerly one 

 present on each visceral arch. It is pointed out that were this so, it 

 would afford a theory of the origin of the vertebrate limb, which 

 would be supported by much of the evidence brought forward by the 

 supporters of the Gegenbaur view, and which at the same time would 

 avoid the most important difficulties in the way of this view. 



" The Thermal Expansion of Pure Mckel and Cobalt." By A. E. 

 Tutton, B.Sc. Communicated by Professor Tilden, D.Sc, 

 F.E.S. Eeceived April 18,— Bead May 4, 1899. 



(Abstract.) 



The author has carried out a series of re-determinations of the co- 

 efficients of thermal expansion of these two metals with the aid of the 

 interference clilatometer described in a former communication to the 



VOL. LXV. N 



